Semiconductor structure and method of forming the same

ABSTRACT

A semiconductor structure and a method for forming the same are provided. The semiconductor structure includes: a substrate; a fin structure protruding from the substrate, the fin structure extending along a first direction; isolation features disposed on both sides of the fin structure; a gate structure over the fin structure and extending on the isolation features along a second direction perpendicular to the first direction; and wherein the gate structure includes a first segment and a second segment, the second segment being over the first segment and including a greater dimension in the first direction than that of the first segment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of prior-filed application Ser. No.14/689,786, filed Apr. 17, 2015, under 35 U.S.C. 120.

FIELD

The present disclosure relates generally to a semiconductor structureand more particularly relates to a three dimensional transistor.

BACKGROUND

For integrated circuit manufacturers, one of the several strategiesemployed for improving integration level and reducing manufacturing costof integration circuits is the introduction of multi-gate devices (e.g.,a multiple gate field-effect transistor, which incorporates more thanone gate into a single transistor). The multi-gate device, such as a finfield effect transistor (FinFET), is proposed to replace theconventional planar MOSFET since it is getting harder and harder toreduce the physical dimension of the conventional planar MOSFET.

However, according to conventional fabrication techniques, it is verylikely that the gate structures of FinFETs would get in direct contactwith one another as the size of ICs get smaller. Accordingly, what isneeded are FinFET structures that can prevent the shorting of adjacentmetal gates.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a schematic diagram illustrating a semiconductor structure inaccordance with one embodiment of the present disclosure.

FIG. 2 is a cross-sectional view illustrating a semiconductor structurein accordance with FIG. 1 of the present disclosure.

FIG. 3 is a cross-sectional view illustrating a semiconductor structurein accordance with FIG. 1 of the present disclosure.

FIG. 4A is a schematic diagram illustrating a semiconductor structure inaccordance with one embodiment of the present disclosure.

FIG. 4B is a schematic diagram illustrating a semiconductor structure inaccordance with one embodiment of the present disclosure.

FIGS. 5A-5J schematically illustrate a method of forming a semiconductorstructure in accordance with one embodiment of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The making and using of the embodiments are discussed in detail below.It should be appreciated, however, that the present invention providesmany applicable inventive concepts that can be embodied in a widevariety of specific contexts. The specific embodiments discussed aremerely illustrative of specific ways to make and use the invention, anddo not limit the scope of the invention.

According to existing metal gate fabrication techniques used infabricating MOSFETs, FinFETs or other types of field effect transistors,an uniform undercut profile for a metal gate structure (e.g., a metalgate structure having vertical sidewalls rather than outwardly slantedsidewalls) is not easily obtainable due to difficulty in uniform etchingof polysilicon gate structure. Accordingly, there often exists a problemof metal gate footing, which would cause the shorting of adjacent metalgates. In addition, for a FinFET device, conventional metal gatefabrication techniques would often result in an undesirable fin topdamage caused by some etching processes. Accordingly, what is needed areFinFET structures that can prevent the shorting of adjacent metal gatesand also prevent the undesirable fin top damage.

In order to solve the aforementioned problems, the present disclosureprovides a semiconductor structure (and a method for forming the same)with indented gate segment at the interface with the substrate/fin(e.g., with the presence of an uniform undercut at the interface) so asto prevent the shorting of adjacent metal gates. Furthermore, theproblem of an undesirable fin top damage can also be solved by means ofan additional layer with relatively high conductivity according to thepresent disclosure.

In reference to the drawings, FIG. 1 is a schematic diagram illustratinga semiconductor structure 100 in accordance with one embodiment of thepresent disclosure. The semiconductor structure 100 may be a multi-gatenon-planar field effect transistor (e.g., FinFET). As shown in FIG. 1,the semiconductor structure 100 includes: a substrate 101, a finstructure 102, a gate structure 103, a high K dielectric layer 104,dielectric sidewalls 105 a and 105 b, an inter-layer dielectric (ILD)layer 106 and isolation features 107.

The substrate 101 is an underlying layer which provides support to thesemiconductor structure 100. The substrate 101 may be a bulk siliconsubstrate, epitaxial silicon substrate, silicon germanium substrate,silicon carbide substrate, silicon germanium substrate, or other groupIII-V compound substrate.

The fin structure 102 is formed as a thin, planar structure protrudingfrom the substrate 101 and extends along a first direction (x directionin FIG. 1) and may be formed of the same material as the substrate 101.The fin structure 102 may include a source region, a drain region and achannel region (not shown). The source region and the drain region areseparated by the channel region, which is wrapped-around by the gatestructure 103. The width of the gate structure 103 (measured in the xdirection in FIG. 1) determines the effective channel length of thesemiconductor structure 100. The wrap-around gate structure 103 providesa better electrical control and thus helps in reducing the leakagecurrent and overcoming other short-channel effects.

The isolation feature 107 disposed on both sides of the fin structure102 may be shallow trench isolation (STI) features that can preventelectrical current leakage between adjacent fin structures 102 (oradjacent semiconductor structures 100). The isolation feature 107 may beformed by the following steps: etching a pattern of trenches in thesubstrate 101, depositing one or more dielectric materials (such assilicon dioxide) to fill the trenches, and removing the excessdielectric thereby exposing the top of the fin structure 102. Theisolation features 107 can be formed by, wet or dry thermal oxidation,physical vapor deposition (PVD), atomic layer deposition (ALD), highdensity plasma CVD (HDPCVD), other suitable methods, and/or combinationsthereof. In one embodiment, the isolation feature 107 may be formed ofsilicon dioxide. The isolation feature 107 may have a multilayerstructure, for example, a thermal oxide liner layer with silicon oxideor silicon nitride formed over the liner.

The gate structure (gate electrode) 103 is arranged on the isolationfeatures 107 to straddle the fin structure 102 and runs in a seconddirection (y direction in FIG. 1) substantially perpendicular to thefirst direction, along which the fin structure 102 extends (x directionin FIG. 1). The gate structure 103 can be formed of any suitable gateelectrode material. In one exemplary embodiment, the gate structure 103could be a metal gate electrode formed by, such as, but not limited to,copper, ruthenium, palladium, platinum, cobalt, nickel, ruthenium oxide,tungsten, aluminum, titanium, tantalum, titanium nitride, tantalumnitride, hafnium, zirconium, a metal carbide, or a conductive metaloxide. It should also be appreciated that the gate structure 103 needsnot be a single material, but could include a composite stack of thinfilms.

The high K dielectric layer 104 is disposed between the fin structure102 and the gate structure 103 and disposed between the isolationfeatures 107 and the gate structure 103. The high K dielectric layer 104can be formed from any gate dielectric material. In one embodiment, thehigh K dielectric layer 104 include a silicon dioxide, siliconoxynitride or a silicon nitride dielectric layer. The thickness of thehigh K dielectric layer 104 may be between about 5 Å to about 20 Å. Thehigh K dielectric layer 104 may have a k value greater than about 7.0,and may include an oxide or a silicate of Hf, Al, Zr, La, Mg, Ba, Ti,Pb, and combinations thereof. Exemplary materials of the high Kdielectric layer 104 include MgO_(x), BaTi_(x)O_(y),BaSr_(x)Ti_(y)O_(z), PbTi_(x)O_(y), PbZr_(x)Ti_(y)O_(z), and the like,with values X, Y, and Z being between 0 and 1. The high K dielectriclayer 104 may be formed by Molecular-Beam Deposition (MBD), Atomic LayerDeposition (ALD), Physical Vapor Deposition (PVD), and the like.

The ILD layer 106 is used for electrical separation and features lowdielectric constant k to minimize capacitive coupling. The ILD layer 106may be formed by chemical vapor deposition (CVD), high density plasmaCVD (HDP-CVD), spin-on deposition, physical vapor deposition (PVD orsputtering), or other suitable methods. The ILD layer 106 may includesilicon oxide, silicon oxynitride, a low-k material, and/or othersuitable dielectric. The ILD layer 106 may be formed on and surroundingthe gate structure 103 and the fin structure 102.

According to one embodiment of the present disclosure, the gatestructure 103 includes a first segment 103 a and a second segment 103 b.The second segment 103 b is over the first segment 103 a and isseparated from the underlying fin structure 102 and isolation features107 by the first segment 103 a. The first segment 103 a and the secondsegment 103 b of the gate structure 103 may be formed using the samematerial or fabrication process. In one embodiment, the first segment103 a is sandwiched between dielectric sidewalls 105 a and the secondsegment 103 b is sandwiched between dielectric sidewalls 105 b. In oneembodiment, the dielectric sidewalls 105 a may be silicon oxidesidewalls doped with group III or group V elements (group IIIelement/dopant may include arsenic (As), phosphorous (P) or antimony(Sb) whereas group III dopants may include boron (B)). The concentrationof the group III or group V element in the dielectric sidewalls 105 aranges from about 1E19 to about 1E22 atoms/cm³. In one embodiment, thedielectric sidewalls 105 b may be silicon oxide sidewalls doped withcarbon or nitrogen with a concentration ranging from about 5E18 to about1E21 atoms/cm³.

According to one embodiment of the present disclosure, the first segment103 a and the second segment 103 b are such configured that the secondsegment 103 b has a greater dimension measured in the first direction (xdirection in FIG. 1) than that of the first segment 103 a, or, to put itin another way, the width of the first segment 103 a is smaller than thewidth of the second segment 103 b. Such a configuration/arrangement canefficiently prevent a gate footing profile, which causes shortingbetween adjacent gate structures (e.g., adjacent gates straddling thesame fin structure). Gate footing is a common defect in metal gatefabrication process.

FIG. 2, which is a cross-sectional view illustrating the semiconductorstructure 100 of FIG. 1 obtained from the plane crossing line A-A inFIG. 1, shows in detail the geometrical configuration of the firstsegment 103 a and the second segment 103 b. In FIG. 2, the parameterW_(103a) designates the width of the first segment 103 a and theparameter W_(103b) designates the width of the second segment 103 b.Since the parameter W_(103b) is selected to be greater than theparameter W_(103a), an undercut region (not shown) would appear on bothsides of the first segment 103 a. As stated above, the undercut regioncan prevent shorting between adjacent gate structures. In oneembodiment, the parameters W_(103a) and W_(103b) are such selected thatW_(103b) is greater than W_(103a) by about 2 nm to about 6 nm. In oneembodiment, the parameters W_(103a) and W_(103b) are such selected thatW_(103b) is greater than W_(103a) by about 3 nm to about 5 nm. In oneembodiment, the parameters W_(103a) and W_(103b) are such selected thatW_(103b) is greater than W_(103a) by about 4 nm. The parameter W_(diff)designates the width difference between the first segment 103 a and thesecond segment 103 b on one side of the first segment 103 a (namely thewidth of the undercut region). In one embodiment, the parameter W_(diff)ranges from about 1 nm to about 3 nm. In one embodiment, the parameterW_(diff) is about 2 nm. Additionally, the parameter H_(103a) designatesthe height of the first segment 103 a and the parameter H_(103b)designates the height of the second segment 103 b. In one embodiment,the parameter H_(103a) ranges from about 5 nm to about 50 nm. In oneembodiment, the parameter H_(103a) ranges from about 5 nm to about 20nm. In one embodiment, the parameter H_(103a) ranges from about 5 nm toabout 10 nm. In one embodiment, the ratio of H_(103b) to H_(103a) rangesfrom about 2 to about 6. In one embodiment, the ratio of H_(103b) toH_(103a) ranges from about 3 to about 5. In one embodiment, the ratio ofH_(103b) to H_(103a) is about 4.

FIG. 3 is a cross-sectional view illustrating the semiconductorstructure 100 of FIG. 1 obtained from the plane crossing line B-B inFIG. 1 (note that the line B-B in FIG. 1 runs through the area rightabove the dielectric sidewall 105 b on the right). FIG. 3 shows that thedielectric sidewalls 105 a substantially cover the fin structure 102.

FIG. 4A is a schematic diagram illustrating a semiconductor structure400A in accordance with one embodiment of the present disclosure. Thesemiconductor structure 400A includes: a substrate 101, a fin structure102, a gate structure 103, a high K dielectric layer 104, dielectricsidewalls 105 a and 105 b and an ILD layer 106.

The substrate 101 may be a semiconductor-on-insulator (SOI) substrate atleast including a buried oxide (BOX) layer 101 a and a base substratelayer 101 b. The material of the BOX layer 101 a may be SiO₂. Thethickness of the BOX layer 101 a may be greater than 100 nm. The basesubstrate layer 101 b may be formed from silicon, germanium or ΠI-Vcompounds (e.g., silicon carbide, gallium, arsenic indium or indiumphosphide).

The fin structure 102 is a thin, planar structure formed on the BOXlayer 101 a of the substrate 101 and runs in a first direction (xdirection in FIG. 4). The gate structure 103 is formed on the BOX layer101 a of the substrate 101 and is arranged to straddle the fin structure102. The gate structure 103 runs in a second direction (y direction inFIG. 4) substantially perpendicular to the first direction (x directionin FIG. 4). The high K dielectric layer 104 is disposed between the finstructure 102 and the gate structure 103 and between the BOX layer 101 aand the gate structure 103. The gate structure 103 includes a firstsegment 103 a and a second segment 103 b over the first segment 103 a.The first segment 103 a is sandwiched between dielectric sidewalls 105 aand the second segment 103 b is sandwiched between dielectric sidewalls105 b. In one embodiment, the dielectric sidewalls 105 a may be siliconsidewalls doped with group III or group V elements and the dielectricsidewalls 105 b may be may be silicon sidewalls doped with carbon ornitrogen. The ILD layer 106 of the semiconductor structure 400A may beformed on and surrounding the gate structure 103 and the fin structure102.

The fin structure 102 of the semiconductor structure 400A differs fromthe fin structure 102 of the semiconductor structure 100 mainly in thatthe former is formed on the BOX layer 101 a over the substrate 101 ofthe semiconductor structure 400A while the latter is penetrating throughthe isolation structure 107 and connecting tithe substrate 101 of thesemiconductor structure 100.

For the semiconductor structure 400A, the width of the first segment 103a is selected to be smaller than the width of the second segment 103 bfor preventing shorting between adjacent gate structures. Since thecross-sectional view of the semiconductor structure 400A is sustainablythe same as that of the semiconductor structure 100, the dimensionparameters of the semiconductor structure 400A noted below will bediscussed with reference to the dimension parameters shown in FIG. 2(namely the cross-sectional view illustrating the semiconductorstructure 100 of FIG. 1 obtained from the plane crossing line A-A inFIG. 1). For semiconductor structure 400A, the width of the secondsegment W_(103b) is greater than the width of the first segment 103 a byabout 2 nm to about 6 nm. In one embodiment, W_(103b) is greater thanW_(103a) by about 3 nm to about 5 nm. In one embodiment, W_(103b) isgreater than W_(103a) by about 4 nm. In one embodiment, the widthdifference between the first segment 103 a and the second segment 103 bon one side of the first segment 103 a (W_(diff)) ranges from about 1 nmto about 3 nm. In one embodiment, the parameter W_(diff) is about 2 nm.In one embodiment, the height of the first segment 103 a H_(103a) rangesfrom about 5 nm to about 50 nm. In one embodiment, H_(103a) ranges fromabout 5 nm to about 20 nm. In one embodiment, the ratio of H_(103b) (theheight of the second segment 103 b) to H_(103a) ranges from about 2 toabout 6. In one embodiment, the ratio of H_(103b) to H_(103a) rangesfrom about 3 to about 5. In one embodiment, the ratio of H_(103b) toH_(103a) is about 4.

FIG. 4B is a schematic diagram illustrating a semiconductor structure400B in accordance with one embodiment of the present disclosure. Thesemiconductor structure 400B includes: a substrate 101, a gate structure103, a high K dielectric layer 104, dielectric sidewalls 105 a and 105 band an ILD layer 106. In one embodiment, the semiconductor structure400B is a planar MOSFET wherein a source region, a drain region and achannel region (not shown) are formed within the substrate 101.

For the semiconductor structure 400B, the width of the first segment 103a is selected to be smaller than the width of the second segment 103 bfor preventing shorting between adjacent gate structures. Similarly,since the cross-sectional view of the semiconductor structure 400B issustainably the same as that of the semiconductor structure 100, thedimension parameters of the semiconductor structure 400B aresubstantially the same as those shown in FIG. 2.

FIGS. 5A-5J schematically illustrate a method of forming a semiconductorstructure (e.g., FinFET) in accordance with one embodiment of thepresent disclosure. In FIG. 5A (operation 5A), a substrate 101 isprovided. The substrate 101 may be an SOI substrate at least including aBOX layer and a base substrate layer (not shown). The material of theBOX layer may be SiO₂ and the base substrate layer may be formed fromsilicon, germanium or ΠI-V compounds. In other embodiments, thesubstrate 101 can be a silicon substrate.

In FIG. 5B (operation 5B), a thin, planar fin structure 102 is formed ona surface of the substrate 101. The fin structure 102 is formed on thesubstrate 101 along a first direction (x direction). In one embodiment,the substrate 101 is an SOI substrate including a top silicon layer, aBOX layer and a base substrate layer (not shown), wherein forming a finstructure 102 on the substrate 101 includes etching away a portion ofthe top silicon layer to define the fin structure 102 (namely remainingportion of the top silicon layer) on the BOX layer. In one embodiment,the substrate 101 is a bulk silicon substrate and forming a finstructure 102 on the substrate 101 includes etching away a portion ofthe substrate 101 to form parallel trenches on the substrate 101 so asto define a fin structure 102 on the substrate 101.

In FIG. 5C (operation 5C), a first silicon layer 105 a′ is formed overthe fin structure 102. In one embodiment, a gate oxide layer (not shown)is formed over the fin structure 102 prior to the formation of the firstsilicon layer 105 a′. Namely the gate oxide layer is first formed overthe fin structure 102 and then the first silicon layer 105 a′ is formedover the gate oxide layer. In one embodiment, the first silicon layer105 a′ is formed under a temperature from about 450 degrees Celsius toabout 650 degrees Celsius, for example, from about 480 degrees Celsiusto about 620 degrees Celsius. In one embodiment, the first silicon layer105 a′ is formed under a pressure from about 0.2 torr to about 5.0 torr.In one embodiment, forming the first silicon layer 105 a′ includesgrowing a silicon layer in-situ doped with a group III or group Velement, wherein an in-situ doping operation includes growing a siliconlayer with dopant gas being concurrently introduced. In one embodiment,the dopant gas includes the group III or group V element. For example,in a CVD operation for in-situ forming the first silicon layer 105 a′,the growing gases include silane (SiH₄), diborane (B₂H₆) and H₂, whereinthe SiH₄ is used for growing the first silicon layer while B₂H₆ providesthe dopant for the first silicon layer. In one embodiment, aconcentration of the group III or group V element in the first siliconlayer 105 a′ ranges from about 1E18 to about 5E22 atoms/cm³. In oneembodiment, a concentration of the group III or group V element in thefirst silicon layer 105 a′ ranges from about 1E19 to about 5E22atoms/cm³. In one embodiment, a concentration of the group III or groupV element in the first silicon layer 105 a′ ranges from about 1E19 toabout 1E22 atoms/cm³. With the concentration of the group III or group Velement in the first silicon layer 105 a′ being from about 1E19 to about1E22 atoms/cm³, the first silicon layer 105 a′ would be a layerrelatively more conductive than a silicon layer without introducingdopants.

In one embodiment, forming the first silicon layer 105 a′ over the finstructure 102 includes forming a first silicon layer 105 a′ ex-situdoped with the group III or group V element, namely the first siliconlayer 105 a′ is formed prior to the doping of the group III or group Velement.

In FIG. 5D (operation 5D), a second silicon layer 105 b′ is formed overthe first silicon layer 105 a′. In some embodiments, the second siliconlayer 105 b′ is a doped silicon layer without typical group III and/orgroup V dopants. For example, the second silicon layer 105 b′ mayinclude carbon or nitrogen. In other embodiments, the second siliconlayer 105 b′ is a doped silicon layer with typical group III and/orgroup V dopants, yet the dopant concentration in the second siliconlayer 105 b′ is measurably lower than that in the first silicon layer105 a′. In one embodiment, the temperature and the pressure used fordepositing the first silicon layer 105 a′ and the second silicon layer105 b′ are substantially the same. In one embodiment, the second siliconlayer 105 b′ is formed under a temperature from about 450 degreesCelsius to about 650 degrees Celsius, for example, from about 480degrees Celsius to about 620 degrees Celsius. In one embodiment, thesecond silicon layer 105 b′ is formed under a pressure from about 0.2torr to about 5.0 torr. Gases including SiH₄, C₂H₄ and/or H₂ are alsointroduced during the formation of the second silicon layer 105 b′. Inone embodiment, forming the second silicon layer 105 b′ includes growinga silicon layer in-situ (or ex-situ, namely the implant operation ofcarbon and/or nitrogen performed after the formation of a silicon layer)doped with a carbon or nitrogen. In one embodiment, a concentration ofcarbon and/or nitrogen in the second silicon layer 105 b′ ranges fromabout 1E18 to about 5E22 atoms/cm³. In one embodiment, a concentrationof the carbon and/or nitrogen in the second silicon layer 105 b′ rangesfrom about 1E19 to about 5E22 atoms/cm³. In one embodiment, aconcentration of the carbon and/or nitrogen in the second silicon layer105 b′ ranges from about 5E18 to about 1E21 atoms/cm³. The first siliconlayer 105 a′ and the second silicon layer 105 b′ are such formed thatthe oxidation rate of the first silicon layer 105 a′ is substantiallygreater than that of the second silicon layer 105 b′.

In FIG. 5E (operation 5E), the first silicon layer 105 a′ and the secondsilicon layer 105 b′ are patterned to form a dummy gate stack (the 105a′/105 b′ stack in FIG. 5E) over the fin structure 102 and extendingalong a second direction perpendicular to the first direction alongwhich the fin structure 102 extends. Patterning the first silicon layer105 a′ and the second silicon layer 105 b′ includes etching away aportion of the first silicon layer 105 a′ and the second silicon layer105 b′ such that the remaining portion forms a dummy gate stack (the 105a′/105 b′ stack in FIG. 5E) over the fin structure 102. The etchingprocess may include wet etching and dry etching. For the wet etchingprocess, the exposed surface of a layer to be etched is dissolved whenimmersed in a bath of liquid-phase (“wet”) etchants, which must beagitated to achieve good process control, wherein wet etchants areusually isotropic. For the dry etching process, the exposed surface of asubstrate is bombarded by ions (usually a plasma of reactive gases suchas fluorocarbons, oxygen, chlorine, boron trichloride; sometimes withaddition of nitrogen, argon, helium and other gases). Unlike with manyof the wet chemical etchants used in wet etching, the dry etchingprocess typically etches directionally or anisotropically. The dryetching process includes ion milling (sputter etching), reactive-ionetching (RIE), deep reactive-ion etching (DRIE) and so on. In someembodiments, a dry etching operation is followed by a wet etchingoperation for cleaning the bottom corners between patterns.

In FIG. 5F (operation 5F), the dummy gate stack (the 105 a′/105 b′stack) is further oxidized. In one embodiment, the dummy gate stack (the105 a′/105 b′ stack) is oxidized under a temperature from about 400degrees Celsius to about 1000 degrees Celsius, preferably from 500degrees Celsius to 950 degrees Celsius. In one embodiment, the dummygate stack (the 105 a′/105 b′ stack) is oxidized under a pressure fromabout 1 torr to about 120 torr, preferably from about 2 torr to about100 torr. The oxidation process is performed with the introduction ofH₂/O₂ with the percentage of H₂ being from about 0.4% to about 40%,preferably from about 0.5% to about 33%. Since the oxidation rate of thefirst silicon layer 105 a′ is substantially greater than that of thesecond silicon layer 105 b′, the oxidized portion of the first siliconlayer 105 a′ (namely the oxidized portion 105 a, corresponding to thedielectric sidewalls 105 a in FIG. 1) would be thicker/wider (measuredin the x direction) than that of the oxidized portion of the secondsilicon layer 105 b′ (namely the oxidized portion 105 b, correspondingto the dielectric sidewalls 105 b in FIG. 1). In other words, theun-oxidized portion of the first silicon layer 105 a′ (corresponding tothe first segment 103 a in FIGS. 1 and 2) would be narrower than that ofthe un-oxidized portion of the second silicon layer 105 b′(corresponding to the second segment 103 b in FIGS. 1 and 2). Since theconfiguration of FIG. 5F is substantially the same as that of FIG. 2,the dimension parameters of FIG. 5F noted below are to be discussed withreference to the dimension parameters shown in FIG. 2. In FIG. 5F, thewidth of the un-oxidized portion of the second silicon layer 105 b′(corresponding to the second segment 103 b in FIG. 2) is greater thanthe width of the un-oxidized portion of the first silicon layer 105 a′(corresponding to the first segment 103 a in FIG. 2) by about 2 nm toabout 6 nm. In one embodiment, the width of the un-oxidized portion ofthe second silicon layer 105 b′ is greater than the width of theun-oxidized portion of the first silicon layer 105 a′ by about 3 nm toabout 5 nm. In one embodiment, the width of the un-oxidized portion ofthe second silicon layer 105 b′ is greater than the width of theun-oxidized portion of the first silicon layer 105 a′ by about 4 nm. Inone embodiment, the difference between the width of the un-oxidizedportion of the first silicon layer 105 a′ and the un-oxidized portion ofthe second silicon layer 105 b′ on one side (of the un-oxidized portionof the first silicon layer 105 a′) ranges from about 1 nm to about 3 nm,preferably the width difference is about 2 nm. In one embodiment, theheight of the first silicon layer 105 a′ ranges from about 5 nm to about50 nm. In one embodiment, the height of the first silicon layer 105 a′ranges from about 5 nm to about 20 nm. In one embodiment, the ratio ofthe height of the second silicon layer 105 b′ to the height of the firstsilicon layer 105 a′ ranges from about 2 to about 6. In one embodiment,the ratio of the height of the second silicon layer 105 b′ to the heightof the first silicon layer 105 a′ ranges from about 3 to about 5. In oneembodiment, the ratio of the height of the second silicon layer 105 b′to the height of the first silicon layer 105 a′ is about 4.

In FIG. 5G (operation 5G), an ILD layer is formed over the fin structure102 and around the dummy gate stack (the 105 a′/105 b′ stack). The ILDlayer 106 is used for electrical separation and features low dielectricconstant k to minimize capacitive coupling. The ILD layer 106 mayinclude silicon oxide, silicon oxynitride, a low-k material, and/orother suitable dielectric. In one embodiment, depositing the ILD layerfurther includes using a CMP process to expose the dummy gate stack (the105 a′/105 b′ stack).

In FIG. 5H (operation 5H), the un-oxidized portion of the first siliconlayer 105 a′ and the un-oxidized portion of the second silicon layer 105b′ are etched away to define an opening. Note that the oxidized portion105 a and the oxidized portion 105 b remains substantially intact due tothe high selectivity of the etching process, namely only the un-oxidizedportion, or polysilicon, would be etched away. Since the original gateoxide can be removed in this operation, the fin top is directly exposedto the dry/wet etchants. However, since the first silicon layer 105 a′is a conductive layer, the underlying fin structure 102 can be protectedfrom damage caused by dry etch process (plasma etching) or spin processduring wet etching (e.g., damage caused by electrostatic chargeaccumulation).

In FIG. 5I (operation 5I), a thin high K dielectric layer 104 isdisposed in the opening and on the fin structure 102. The high Kdielectric layer 104 can be formed from any gate dielectric material. Inone embodiment, the high K dielectric layer 104 includes a silicondioxide, silicon oxynitride or a silicon nitride dielectric layer. Thethickness of the high K dielectric layer 104 may be between about 5 Å toabout 20 Å. The high K dielectric layer 104 may have a k value greaterthan about 7.0.

In FIG. 5J (operation 5J), a metal is deposited within the opening toform a gate structure 103. Metal deposition processes, such as CVD, PVD,ALD, sputtering, electroplating, or electroless plating, may be used todeposit the gate structure 103. The gate structure 103 could be a metalgate electrode, such as, but not limited to, copper, ruthenium,palladium, platinum, cobalt, nickel, ruthenium oxide, tungsten,aluminum, titanium, tantalum, titanium nitride, tantalum nitride,hafnium, zirconium, a metal carbide, or a conductive metal oxide. Itshould also be appreciated that the gate structure 103 needs not be asingle material, but could include a composite stack of thin films. Inone embodiment, depositing metal within the opening further includesusing a CMP process to planarize the deposited metal.

Accordingly, the semiconductor structure fabricated based on the methodshown in FIGS. 5A-5J would advantageously have a metal gate structure103 with indented gate segment at the interface with the substrate/finstructure (e.g., an uniform undercut at the interface). Hence, theshorting of adjacent metal gates can be efficiently prevented. Oncontrast, for conventional fabrication processes, the metal gatestructure would have outwardly slanted sidewalls (footing profile) dueto non-uniform etching of polysilicon gate structure.

One embodiment of the present disclosure provides a semiconductorstructure, comprising: a substrate; a fin structure protruding from thesubstrate, the fin structure extending along a first direction;isolation features disposed on both sides of the fin structure; a gatestructure over the fin structure and extending on the isolation featuresalong a second direction perpendicular to the first direction; andwherein the gate structure includes a first segment and a secondsegment, the second segment being over the first segment and including agreater dimension in the first direction than that of the first segment.

In one embodiment, a difference between the dimension of the firstsegment and the second segment ranges from about 2 nm to about 6 nm.

In one embodiment, the semiconductor structure further includes a high Kdielectric layer between the fin structure and the first segment of thegate structure.

In one embodiment, the gate structure includes a metal gate.

In one embodiment, a height of the first segment ranges from about 5 nmto about 50 nm.

In one embodiment, the ratio of a height of the second segment to aheight of the first segment ranges from about 2 to about 6.

In one embodiment, the first segment is sandwiched by dielectricsidewalls doped with group III or group V elements.

One embodiment of the present disclosure provides a semiconductorstructure, including: a semiconductor substrate; a gate structureextending over the semiconductor substrate, wherein the gate structureincludes a first segment and a second segment, the second segment beingover the first segment and including a greater dimension in a directionperpendicular to which the gate structure extends than that of the firstsegment.

In one embodiment, a difference on one side between the dimension of thefirst segment and the second segment ranges from about 1 nm to about 3nm.

In one embodiment, the ratio of a height of the second segment to aheight of the first segment ranges from about 2 to about 6.

One embodiment of the present disclosure provides a method for forming asemiconductor structure including: forming a fin structure along a firstdirection on a semiconductor substrate; depositing a first layer overthe semiconductor substrate and the fin structure; and depositing asecond layer over the first layer, wherein depositing the first layerincludes growing a silicon layer in-situ doped with a group III or groupV element.

In one embodiment, the group III element includes boron.

In one embodiment, a concentration of the group III or group V elementin the first silicon layer ranges from about 1E19 to about 1E22atoms/cm3.

In one embodiment, depositing the second layer includes growing asilicon layer doped with carbon or nitrogen.

In one embodiment, the temperature and the pressure used for depositingthe first layer and the second layer are substantially the same.

In one embodiment, the method further includes patterning the firstlayer and the second layer to form a dummy gate stack over the finstructure and extending along a second direction perpendicular to thefirst direction.

In one embodiment, the method further includes oxidizing the first layerand the second layer of the dummy gate stack.

In one embodiment, the oxidizing is performed under a pressure rangingfrom about 2 to about 100 torr.

In one embodiment, the oxidation rate of the first layer is greater thanthat of the second layer.

In one embodiment, the method further includes removing an un-oxidizedportion of the dummy gate stack.

The methods and features of this disclosure have been sufficientlydescribed in the above examples and descriptions. It should beunderstood that any modifications or changes without departing from thespirit of the disclosure are intended to be covered in the protectionscope of the disclosure.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, and composition of matter, means, methods and stepsdescribed in the specification. As those skilled in the art will readilyappreciate from the disclosure of the present disclosure, processes,machines, manufacture, composition of matter, means, methods or stepspresently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present disclosure. Accordingly, the appended claims are intended toinclude within their scope such as processes, machines, manufacture,compositions of matter, means, methods or steps/operations. In addition,each claim constitutes a separate embodiment, and the combination ofvarious claims and embodiments are within the scope of the disclosure.

What is claimed is:
 1. A semiconductor structure, comprising: asubstrate; a fin structure protruding from the substrate, the finstructure extending along a first direction; isolation features disposedon both sides of the fin structure; a gate structure over the finstructure and extending on the isolation features along a seconddirection perpendicular to the first direction; and wherein the gatestructure comprises a first segment and a second segment over the firstsegment, and the first segment is sandwiched by a first dielectricsidewalls doped with group III or group V elements.
 2. The semiconductorstructure of claim 1, wherein the second segment is sandwiched by asecond dielectric sidewalls doped with carbon or nitrogen.
 3. Thesemiconductor structure of claim 1, further comprising a high Kdielectric layer between the fin structure and the first segment of thegate structure.
 4. The semiconductor structure of claim 1, wherein thefirst segment and the second segment of the gate structure comprisesmetallic materials.
 5. The semiconductor structure of claim 1, wherein aheight of the first segment ranges from about 5 nm to about 50 nm. 6.The semiconductor structure of claim 1, wherein the ratio of a height ofthe second segment to a height of the first segment ranges from about 2to about
 6. 7. The semiconductor structure of claim 1, wherein thesecond segment comprises a greater dimension in the first direction thanthat of the first segment.
 8. The semiconductor structure of claim 7,wherein a difference between the dimension of the first segment and thesecond segment ranges from about 2 nm to about 6 nm.
 9. Thesemiconductor structure of claim 1, wherein the first dielectricsidewalls comprises silicon oxide doped with group III or group Velements.
 10. The semiconductor structure of claim 2, wherein the seconddielectric sidewalls comprises silicon oxide doped with carbon ornitrogen.
 11. A semiconductor device, comprising: a semiconductorsubstrate; a gate structure extending over the semiconductor substrate,wherein the gate structure comprises a first segment and a secondsegment, the second segment being over the first segment and comprisinga greater dimension in a direction perpendicular to which the gatestructure extends than that of the first segment, wherein the secondsegment is sandwiched by carbon or nitrogen-doped dielectric sidewalls.12. The semiconductor device of claim 11, wherein the first segment issandwiched by group III or group V element-doped dielectric sidewalls.13. The semiconductor device of claim 12, wherein the group III or groupV element-doped dielectric sidewalls and the carbon or nitrogen-dopeddielectric sidewall comprises silicon oxides.
 14. The semiconductordevice of claim 11, further comprising a high-k dielectric layer betweenthe first segment and the semiconductor substrate.
 15. The semiconductordevice of claim 11, wherein a difference on one side between thedimension of the first segment and the second segment ranges from about1 nm to about 3 nm.
 16. The semiconductor device of claim 11, whereinthe ratio of a height of the second segment to a height of the firstsegment ranges from about 2 to about
 6. 17. A FinFET structure,comprising: a semiconductor fin; a metal gate orthogonally over thesemiconductor fin, the metal gate comprising: a metal electrode having anarrower section at in proximity to the semiconductor fin and a widersection over the narrower section; a first dielectric sidewallsurrounding the narrower section of the metal electrode; a seconddielectric sidewall surrounding the wider section of the metalelectrode, wherein the first dielectric sidewall is wider than thesecond dielectric sidewall; and a dielectric layer surrounding the metalgate, the first dielectric sidewall, and the second dielectric sidewall.18. The FinFET structure of claim 17, further comprising a high-kdielectric between the semiconductor fin and the narrower section,surrounded by the first dielectric sidewall.
 19. The FinFET structure ofclaim 17, wherein the first dielectric sidewall comprises group III orgroup V element-doped oxides.
 20. The FinFET structure of claim 17,wherein the second dielectric sidewall comprises carbon ornitrogen-doped oxides.